Whereas the number of refugees assisted by the Office of the United Nations High Commissioner for Refugees (UNHCR) had fallen to 10.6 million by the end of 2002, the number of internally displaced persons was estimated to be about 20–25 million at the same date. Internally displaced persons not only outnumber, by far, refugees, they also raise some of the most urgent human rights and humanitarian problems of our time and present a serious challenge to prevailing conceptions of sovereignty and intervention. They can be found on all continents, but especially in Sub-Saharan Africa, the Middle East, the former Yugoslavia and in the republics of the former Soviet Union. Some countries are particularly affected, such as Sudan with an estimated 4 million internally displaced. In 2003, other countries such as Colombia, the Democratic Republic of Congo, Iraq and Turkey hosted up to, or even more than, a million internally displaced persons each. The refugee definition contained in the 1951 Refugee Convention, as modified by the 1967 Protocol, indicates that internally displaced persons are not refugees because they are still within their country of origin. They have not crossed a frontier, which is a precondition of refugeehood.
Until the beginning of the 1990s, internally displaced persons were defined negatively: they were people who had fled their homes, but who were not refugees (having remained within their country). It is only recently that some efforts have been made to devise a comprehensive definition of internally displaced persons.